Photochemistry and Thermolysis of the 1,1-Diphenylindene System

Abstract

<p> 1,1-Diarylindenes rearrange when heated or irradiated to give 1,2- and 2,3-diarylindenes. Three 1,1-diarylindenes were synthesized and used in an attempt to probe the transition states for phenyl migration in both the ground and excited state. The three indenes synthesized were: 1-(p-bromophenyl)-1-phenylindene, 1-(p-cyanophenyl)-1-phenylindene and 1-(p-methoxyphenyl)-1-phenylindene.</p> <p> On direct irradiation, the migratory aptitudes were -C6H4CN-p, 98%; -C6H4OCH3-p, 95%; and -C6H4Br-p, 86%. On heating (258°C) the following aptitudes were found: -C6H4CN-p, 82%; -C6H4OCH3-p, 48%; and -C6H4Br-p, 53%. The products were synthesized independently, and the product ratios obtained upon reaction were measured from the n.m.r. of mixtures, and by v.p.c. A charge transfer mechanism was put forward to explain the preference for migration of the substituted group in the excited state. Such charge transfer contributions are known to increase the rates of quenching in donor-acceptor pairs. A radical-type transition state was postulated as being present in the ground state migration. The rate constant for excited state rearrangement was calculated to be 4.9 x 10^9 sec^-1 .</p>